The entire disclosure of Japanese Patent Application No. 2005-315138, filed on Oct. 28, 2005, and Japanese Patent Application No. 2006-276856, filed on Oct. 10, 2006, is expressly incorporated by reference herein.
1. Technical Field
The present invention relates to a method for forming a mark and a liquid ejection apparatus.
2. Related Art
Normally, an electro-optic apparatus such as a liquid crystal display or an electroluminescence display includes a substrate that displays an image. The substrate has an identification code (for example, a two-dimensional code) including product information regarding the name of the manufacturer and the product number, for purposes of quality control and production control. The identification code includes a plurality of dots formed by, for example, colored thin films or recesses. The dots are arranged in a predetermined pattern so that the identification code can be identified in accordance with the arrangement pattern of the dots.
As a method for forming an identification code, JP-A-11-77340 discloses a laser sputtering method and JP-A-2003-127537 discloses a waterjet method. In the laser sputtering method, a code pattern is formed through sputtering by radiating a laser beam onto a metal foil. In the waterjet method, dots are marked on a substrate by ejecting water containing abrasive onto the substrate.
However, in the laser sputtering method, the interval between the metal foil and the substrate must be adjusted to several micrometers to several tens of micrometers in order to form each dot in a desired size. The substrate and the metal foil thus must have extremely flat surfaces and adjustment of the interval between the substrate and the metal foil must be carried out with accuracy on the order of micrometers. This limits application of the method to a restricted range of substrate, and the use of the method is limited. In the waterjet method, the substrate may be contaminated by water, dust, and the abrasive that are splashed when the identification code is formed.
In order to solve these problems, an inkjet method has been focused on as an alternative method for forming an identification code. In the inkjet method, dots are provided on a substrate by ejecting droplets of liquid containing metal particles from nozzles of an ejection head onto the substrate. The droplets are then dried to provide the dots. The method thus can be applied to a relatively wide range of substrate materials. Further, the method prevents contamination of the substrate caused by formation of the identification code.
However, the inkjet method may cause the following problem in correspondence with the surface condition of a substrate or surface tension of a droplet. Specifically, immediately after having been received by a substrate, a droplet starts to spread wet on the surface of the substrate. Thus, if the time necessary for the droplet to be dried is excessively long (for example, 100 milliseconds or longer), the droplet may spread excessively on the surface of the substrate and overflow from the corresponding data cell. This makes the code pattern unreadable, which causes loss of the information regarding the substrate.
This problem may be avoided by radiating a laser beam onto the droplet on the substrate and instantly drying the droplet. However, in a typical liquid ejection head, the interval between the nozzles and the surface of the substrate is maintained at several millimeters to improve position accuracy of reception of an ejected droplet by the substrate. The laser beam thus must be radiated onto the droplet toward the narrow distance between the ejection head and the substrate immediately after the droplet has been received by the substrate. That is, it is necessary to greatly incline the optical axis of the laser beam with respect to a normal direction of the surface of the substrate. Accordingly, the optical cross section of the laser beam, or a beam spot, with respect to the surface of the substrate or the droplet becomes excessively large on the surface of the substrate. This may lower the radiation intensity of the laser beam or the position accuracy of radiation of the laser beam.